// Copyright (c) 2018 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#ifndef BITCOIN_SPAN_H
#define BITCOIN_SPAN_H

#include <algorithm>
#include <cstddef>
#include <type_traits>

/** A Span is an object that can refer to a contiguous sequence of objects.
 *
 * It implements a subset of C++20's std::span.
 */
template <typename C> class Span {
    C *m_data;
    std::ptrdiff_t m_size;

public:
    constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
    constexpr Span(C *data, std::ptrdiff_t size) noexcept
        : m_data(data), m_size(size) {}
    constexpr Span(C *data, C *end) noexcept
        : m_data(data), m_size(end - data) {}

    constexpr C *data() const noexcept { return m_data; }
    constexpr C *begin() const noexcept { return m_data; }
    constexpr C *end() const noexcept { return m_data + m_size; }
    constexpr std::ptrdiff_t size() const noexcept { return m_size; }
    constexpr C &operator[](std::ptrdiff_t pos) const noexcept {
        return m_data[pos];
    }

    constexpr Span<C> subspan(std::ptrdiff_t offset) const noexcept {
        return Span<C>(m_data + offset, m_size - offset);
    }
    constexpr Span<C> subspan(std::ptrdiff_t offset, std::ptrdiff_t count) const
        noexcept {
        return Span<C>(m_data + offset, count);
    }
    constexpr Span<C> first(std::ptrdiff_t count) const noexcept {
        return Span<C>(m_data, count);
    }
    constexpr Span<C> last(std::ptrdiff_t count) const noexcept {
        return Span<C>(m_data + m_size - count, count);
    }

    friend constexpr bool operator==(const Span &a, const Span &b) noexcept {
        return a.size() == b.size() &&
               std::equal(a.begin(), a.end(), b.begin());
    }
    friend constexpr bool operator!=(const Span &a, const Span &b) noexcept {
        return !(a == b);
    }
    friend constexpr bool operator<(const Span &a, const Span &b) noexcept {
        return std::lexicographical_compare(a.begin(), a.end(), b.begin(),
                                            b.end());
    }
    friend constexpr bool operator<=(const Span &a, const Span &b) noexcept {
        return !(b < a);
    }
    friend constexpr bool operator>(const Span &a, const Span &b) noexcept {
        return (b < a);
    }
    friend constexpr bool operator>=(const Span &a, const Span &b) noexcept {
        return !(a < b);
    }
};

/** Create a span to a container exposing data() and size().
 *
 * This correctly deals with constness: the returned Span's element type will be
 * whatever data() returns a pointer to. If either the passed container is
 * const, or its element type is const, the resulting span will have a const
 * element type.
 *
 * std::span will have a constructor that implements this functionality
 * directly.
 */
template <typename A, int N> constexpr Span<A> MakeSpan(A (&a)[N]) {
    return Span<A>(a, N);
}

template <typename V>
constexpr Span<
    typename std::remove_pointer<decltype(std::declval<V>().data())>::type>
MakeSpan(V &v) {
    return Span<
        typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(
        v.data(), v.size());
}

#endif // BITCOIN_SPAN_H
